1. Field of the Invention
The present invention relates to an optical filter tuning apparatus and optical filtering method.
2. Description of Related Art
In order to filter light emitted from an optical fibre a chirped dichromated gelatin (DCG) filter may be used. A filter of this type is described in a paper entitled "Position--tunable holographic filters in dichromated gelatin for use in single - mode - fiber demultiplexers", Optical Letters, Vol. 10, page 303, Jun. 1985. Light is normally directed from an input optical fibre onto the chirped grating filter using a lens arrangement. As the filter is a chirped grating the wavelength of light reflected therefrom is dependent on the position of incidence of the light from the input fibre. To adjust the filtering characteristics of this filter arrangement, which is described in detail hereinafter, the filter must be moved relative to the lens and input fibre so as to alter the position of incidence. To effect this adjustment regard must be had to very stringent angular tolerances in two planes. The restrictions imposed by the tolerances necessitate the use of an elaborate and expensive support mechanism on which the filter arrangement must be mounted. Hence, it is desirable to provide a filter apparatus and filtering method which reduces the angular tolerances that must be set.